Cyclic dienes, hydrogenation

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Borohydride reduction of NiCl2 in dimethylformamide or dimethyl-acetamide leads to very active catalysts, thought to be homogeneous, for hydrogenation of monoolefins, unsaturated fats, cyclic dienes to monoenes, and saturated aldehydes and ketones (165, 538, 539). Cobaltous chloride systems have also been used (540). 

The characteristics of the hydrogenation of norbornadiene, substituted butadienes and conjugated and cyclic dienes are all very similar. In the case of conjugated dienes, there appears to be hardly any isomerization activity, while in the case of 1,4-dienes an isomerization step to form the corresponding 1,3-diene is assumed prior to hydrogenation. The catalyst behavior changes after the diene has been completely converted to the monoene, whereupon the rhodium catalyst resumes its normaF monoene hydrogenation behavior. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

The heat of hydrogenation of the cyclic diene (8) is very nearly twice that of cyclohexene (7), and the heat of hydrogenation of the three double bonds in a Kekule structure might thus be expected to be of the order of 3 x -120 kJ ( — 28-6 kcal)mol = -360 kJ ( — 85-8 kcal) mol but when real benzene is hydrogenated only —208 kJ (-49-8 kcal) mol are evolved. Real benzene is thus thermodynamically more stable than the hypothetical cyclohexatriene by 151 kJ (36 kcal) mol this compares with only 17 kJ (4 kcal)mol by which a conjugated diene i stabilised, with respect to its analogue in which there is no interaction between the electrons of the double bonds. 

Complexes of carbonic or carboxylic acid anions have been used as hydroformylation catalysts for various alkenes. The bicarbonate complex [Rh(H)2(02COH)(PPr 3)2] as catalyst enabled 1-hexene to be converted to aldehydes using paraformaldehyde as source of hydrogen and carbon monoxide in place of the more usual gas mixture.338 The acetate complex [Rh(OAc)CO(PPh3)2] (74) has been shown to effect the selective hydroformylation of cyclic dienes. The cyclohexadienes gave predominantly dialdehydes, whereas 1,3- and 1,5-cyclooctadiene gave the saturated monoaldehydes.339... 

Since differences in the hydrogenation rates of variously substituted monoalkenes were found over Pt-zeolite catalysts modified by CVD (see Section IV.A.l) these catalysts were anticipated to exhibit regioselectivities in the hydrogenation of dienes. Selective saturation of the less substituted double bonds of linear and cyclic dienes was indeed achieved over Pt-zeolite A and Rh-zeolite A catalysts treated with tetraethoxysilane78 or diethoxydiphenylsilane79,80 (Table 13). 

Stereoselective hydrogenation of 1,3-dienes. Hydrogenation of simple acyclic and cyclic 1,3-dienes catalyzed by (arene)Cr(CO), complexes results in highly stereoselective 1,4-addition of hydrogen to produce (Z)-monoenes. Under these conditions 1,4-dienes are isomerized to 1,3-dienes and then reduced to (Z)-monoenes, but 1,5-dienes are not reduced. ... 

Similar decomposition patterns can be proposed for secondary radicals derived from 2 in the initiating hydrogen abstraction Step 1. Thermal aromatization of 6-ring cyclic dienes containing one exocyclic and one endocyclic double bonds is a facile process at 550°-600°C. The reaction involves fast double bond isomerization to a conjugated cyclohexadiene, followed by dehydrogenation (30, 31,32). 

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